Process devices are used in processing plants to monitor process variables and control industrial processes. Process devices are generally remotely located from a control room and are coupled to process control circuitry in the control room by a process control loop. The process control loop can be a 4-20 mA current loop that powers the process device and provides a communication link between the process device and the process control circuitry. Examples of process devices include process transmitters used to measure temperature, pressure, flow, pH, turbidity, level, or other process variables and various process control devices.
Resistive elements are typically found in process devices. Resistive elements can be process variable sensors, wires, coils, resistive temperature detectors, thermocouples, electrical traces, terminations, and other components of process devices having an electrical resistance. Generally, the condition of these resistive elements tends to degrade over time due to wear caused by use and environmental conditions that results in a decrease in performance. Depending on the resistive element, the decrease in performance may produce inaccurate measurements or cause the process device to fail. As a result, process devices are periodically tested to establish the condition of these resistive elements to determine whether measurements must be compensated or whether the resistive elements must be replaced.
For example, temperature transmitters can use resistive temperature detectors (RTD""s) to measure the temperature of process mediums, such as a process fluid. An RTD is a resistive element that has a temperature-dependent resistance. The RTD is placed in thermal communication with the process fluid and the temperature transmitter injects a current into the RTD. The resultant voltage drop across the RTD is used to calculate the resistance of the RTD. The temperature of the process fluid is determined from the resistance of the RTD. As the condition of the RTD deteriorates, its relationship between resistance and temperature changes thus reducing the accuracy of its measurements. Consequently, temperature transmitters are periodically calibrated to compensate for the changing properties of the RTD.
Present calibration techniques are generally conducted offline. These techniques involve testing the process device at the location of the process device (on-site). One such offline calibration technique is the plunge test. Here, the RTD or temperature sensor is removed from the transmitter and placed in a bath of a known temperature. The output from the sensor is monitored and compared to the actual temperature of the bath to determine the amount of compensation required or calibration factor. The calibration factor is then stored in the processing system of the process device. The calibration factor is used to compensate the output of the process device such that the output accurately represents the temperature of the process medium being measured. Other offline calibration techniques involve injecting the resistive element with a test current and analyzing the response signal produced by the resistive element in response to the test current. These techniques generally utilize an additional power source due to the power limitations of the process device.
The above-mentioned calibration techniques are inadequate because they can require that the device be tested on-site, that the device be disassembled, and that an additional power supply be provided. Furthermore, they are typically time-consuming procedures that can significantly interrupt the operations of the processing plant.
Diagnostic circuitry of a process device is used to detect degradation of a resistive element of the process device while the process device remains online, without the use of an additional power source, and without causing a significant interruption of the operations of the processing plant. The diagnostic circuitry includes a testing circuit and a processing system. The testing circuit is coupled to the resistive element and is configured to apply a test signal to the resistive element. The test signal heats the resistive element and causes the resistive element to generate a response signal. The processing system compares a time constant of the response signal to a corresponding reference to detect degradation of the resistive element.